In the advanced lithography process for the fabrication of semiconductor devices, a light source of shorter wavelength is used for exposure. A subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising. Since the EUV lithography uses a reflecting optical system and a short wavelength light source, the lithography accuracy can be adversely affected even by a slight thermal expansion of each member (e.g., substrate) in the lithographic optical system induced by the heat that has reached there. Accordingly, members like reflecting mirrors, masks, and stages must be made of low expansion materials. Titania-doped quartz glass is known as a typical low expansion material. The addition of a certain amount of titania makes it possible to minimize the thermal expansion of quartz glass.
The EUV lithography members must also have a uniform distribution of low thermal expansion. To gain a uniform distribution of low thermal expansion, it is of the first priority that titania-doped quartz glass have a uniform content of titania. For example, JP-A 2004-315351 discloses titania-doped quartz glass in which a difference between maximum and minimum TiO2 concentrations is less than or equal to 0.06% by weight in a range of 30 mm×30 mm, and a variation (Δn) of refractive index which varies with the TiO2 concentration in quartz glass is less than or equal to 2×10−4 in a range of 30 mm×30 mm.
Also an OH group concentration in titania-doped quartz glass is known as one of the physical properties having impact on the low thermal expansion of titania-doped quartz glass. WO 2005/114328 discloses a quartz glass blank having a mean OH content of 700 to 1,000 wt ppm, wherein the variation of the OH content, averaged over the thickness of the quartz glass blank, is within ±50 ppm in the area of the main functional direction. Then the optical and thermal properties of quartz glass are kept as homogeneous as possible.
JP-A 2005-022954 describes that the fictive temperature of glass is correlated to the extent of a zero expansion temperature range that is a temperature range in which the coefficient of thermal expansion (CTE) of glass becomes substantially zero (0). For the purpose of broadening the zero expansion temperature range, the fictive temperature is preferably up to 950° C., more preferably up to 900° C., and even more preferably up to 850° C. Since a high OH group concentration in glass indicates fast structural relaxation, the manufacture of a glass block having a large diameter enough to have a temperature distribution tends to entail a fictive temperature distribution. Thus the OH group concentration is preferably up to 600 ppm, more preferably up to 400 ppm, and even more preferably up to 200 ppm. In addition, if the OH group concentration varies over a wide range, the structural relaxation time may substantially vary at different positions, inviting a difference in fictive temperature. Thus the variation of the OH group concentration in titania-doped quartz glass is preferably within 50 ppm, more preferably within 30 ppm, and even more preferably within 10 ppm.
As discussed above, the OH group concentration in titania-doped quartz glass has an outstanding impact on low thermal expansion. It is thus important to specify the absolute amount and distribution of the OH group concentration in titania-doped quartz glass. It would be desirable to minimize the variation of the OH group concentration.